Arrays of semiconductor diode lasers are of great interest for applications requiring higher power levels than can be attained with single devices. Development efforts to date have concentrated mainly on fabrication of linear arrays of edge-emitting GaAs/AlGaAs lasers.
Harnagel et. al., Electron. Lett. 22, 605 (1986) report formation of 5 Watt continuous-wave (CW) AlGaAs 1 mm laser diode bars in the form of monolithic 100 to 140 stripped linear arrays. Welch et al., Electron. Lett. 23, 525 (1987) present data on 770 nm, 10 stripe AlGaAs laser arrays with a 2.4 W CW output. Similar success is reported by Harnagel et al., in Electron. Lett. 23, 744 (1987) and Welch et al.. in Electron. Lett. 24, 113 (1988).
While these efforts have met with considerable success, a need for still higher power levels have led those skilled in the art, to attempt, along with continuing work on linear arrays, to achieve still higher output power by stacking and bonding these linear arrays to form composite two-dimensional arrays.
In the stacked and bonded array, linear diode laser arrays and slabs of electrical and heat conducting material forming individual heat sinks, are alternately stacked upon each other. The individual heat sinks are then thermally connected at their edges to a main heat sink from which the heat can be removed by several different techniques. In this arrangement, heat is transferred from the linear diode laser arrays into the individual heat sinks and then transversely along the individual heat sinks to the main heat sink. It is difficult to transfer heat from the linear laser arrays to the main heat sink and, thus, difficult to maintain temperature uniformity across the entire array. Microchannels, through which cooling fluid could be passed, could be incorporated in the individual heat sinks, but only with a significant increase in fabrication complexity.
As an alternate approach to such composite arrays, monolithic two-dimensional arrays consisting of surface-emitting lasers are also being investigated. Several types of surface-emitting GaAs/AlGaAs and GaInAsP/InP diode lasers have been demonstrated. These include lasers with resonant cavities that are normal to the surface [Iga et al., Appl. Phys. Lett. 45, 348 (1984); Uchiyama et al., Electron. Lett. 21, 162 (1985) and Ogura et al., Electron. Lett. 23, 758 (1987)], lasers that incorporate a chemically etched 45.degree. mirror [A. J. Spring Thorpe, Appl. Phys. Lett. 31, 524 (1979)], and lasers that utilize a second-order grating coupler to achieve emission normal to the surface [Carlson et al., Appl. Phys. Lett. 50, 1301 (1987)]. A second approach to the fabrication of monolithic two-dimensional arrays is to couple edge-emitting diode lasers with external mirrors that deflect the radiation from the laser facets by 90.degree.. Liau and Walpole [Appl. Phys. Lett. 50, 528 (1987)] have fabricated such arrays of devices that combine a GaInAsP/InP laser with a parabolic deflector adjacent to one or both facets. The facets and deflectors were formed by selective chemical etching followed by a mass-transport process. Such a mass-transport process is not known for AlGaAs, and therefore, other techniques, such as ion-beam-assisted etching (IBAE) [Windhorn et al., Appl. Phys. Lett. 48, 1675 (1986); Goodhue et al., Gallium Arsenide and Related Compounds 1986, ed. W. T. Lindley (Inst. Phys. Conf. Ser. 83, Briston, 1987, p. 349; Donnelly et al., Appl. Phys. Lett. 51, 1138 (1987)] and ion milling [Yang et al., Appl. Phys. Lett. 49, 1138 (1986)] have been used to form the noncleaved laser facets and adjacent deflectors. Monolithic two-dimensional GaAs/AlGaAs diode laser arrays, in which all of the facets and deflectors were formed by IBAE, have recently been reported by Donnelly et al., above. Although these monolithic techniques hold great promise for large-area high-power laser arrays, they are still in the early stages of development. In some of them, the quality of the mirrors and windows are not as good as the conventional cleaved facets found in linear arrays.